The invention relates to a process for the desulfurization of at least one NOx storage catalytic converter which is arranged in an exhaust pipe of an internal-combustion engine, having the features listed in the preamble of claim 1.
Processes for the desulfurization of NOx storage catalytic converters are known. In these processes, during the desulfurization what are known as regeneration parameters, such as a minimum temperature at the NOx storage catalytic converter and an operating mode of the internal-combustion engine with xcexxe2x89xa61, have to be set.
When the operating mode of the internal-combustion engine involves xcex less than 1, a concentration of reducing gas components, such as CO, HC or H2, predominates in the exhaust gas over a concentration of oxygen (rich atmosphere). At xcex greater than 1, the oxygen concentration is dominant (lean-burn atmosphere). While the internal-combustion engine is operating in a lean-burn atmosphere, SO2 is formed as well as NOx, on account of fluctuating levels of sulfur in the fuel mix being burned. Like the NOx, this SO2, in a lean-burn atmosphere, is absorbed by the NOx storage catalytic converter; the absorption of SO2 may lead to the formation of local inhomogeneities as a result of the formation of sulfate grains. This process leads to a reduction in a catalytically active surface region and in a capacity of the NOx storage catalytic converter and offers a point of attack for corrosive processes which cause permanent damage to the NOx storage catalytic converter.
It is known to measure a degree of sulfurization of the NOx storage catalytic converter on the basis of recording an efficiency with regard to a reduction of NOx (conversion reaction). For example, DE 197 47 222 C1 describes establishing an NOx storage capacity which has been reduced by sulfur poisoning by recording the NOx concentration, which rises relatively rapidly downstream of the NOx storage catalytic converter, in lean-burn phases.
If the degree of sulfurization reaches a threshold value, it is also known to initiate the desulfurization by at least temporarily influencing an operating parameter of the internal-combustion engine (measure). These measures comprise, for example, late ignition, reinjection before or during a combustion operation or a cylinder-selective trimming of the internal-combustion engine, and therefore lead to an increased fuel consumption. These measures are often taken in order to increase an exhaust-gas temperature upstream of the NOx storage catalytic converter and then to reach the minimum temperature at the NOx storage catalytic converter. A drawback of the known processes is that the desulfurization is initiated only on the basis of the efficiency of the conversion reaction, i.e. after the threshold value has been reached, and therefore relatively favorable operating states of the internal-combustion engine which occur in practice and in which the threshold value has not been reached, yet the temperature of the NOx storage catalytic converter is close to the minimum temperature, remain unused. This leads to an increased fuel consumption.
The invention is based on the object of making the desulfurization of the NOx storage catalytic converter significantly more dynamic, i.e. of including not just the degree of sulfurization of the catalytic converter but also the regeneration parameters in order to assess the need for desulfurization.
This object is achieved by the process for the desulfurization of at least one NOx storage catalytic converter arranged in the exhaust pipe of an internal-combustion engine by the features given in claim 1. The fact that the desulfurization is initiated, not only by the degree of sulfurization, but also as a function of a current lambda value and/or a current temperature of the NOx storage catalytic converter means that the desulfurization can be carried out significantly more efficiently, i.e. so that more fuel is saved. Furthermore, it is advantageous that it is possible to maintain a smaller size of the sulfate grains formed by the desulfurization, since overall the desulfurization takes place more frequently.
In a preferred configuration of the process, a characteristic variable which is formed from the degree of sulfurization, the lambda value and the NOx storage catalytic converter temperature is determined in order to assess the need for desulfurization. The characteristic variable lies within a characteristic diagram which can be characterized by means of a ratio of a load to a rotational speed of the internal-combustion engine. This characteristic diagram can be divided into any desired number of regions in which, for example, the threshold value for the desulfurization is reduced as the temperature rises. In this way, it is possible to react very dynamically to given operating states with regard to initiating the desulfurization.
Further preferred configurations of the invention will emerge from the remaining features listed in the subclaims.